Chemical characteristics of the Yamato-type (CY) carbonaceous chondrites

Fifteen elements, including most of the major elements, were quantified using neutron-induced prompt gamma-ray analysis for five Antarctic carbonaceous chondrites with CI affinities and seven with CM affinities. Common among the twelve meteorites is the depletion of volatile elements H and chlorine, showing a positive correlation and being depleted compared to non-Antarctic CI levels. This depletion is not thought to have occurred after the fall on Antarctica, but to have been caused by thermal metamorphism on the parent body. Among the meteorites analyzed in this study, six meteorites (Y-86029, Y 980115 and Y-82162 (with CI affinities), and Y-86720, Y-86789 and B-7904 (with CM affinities)) have previously been proposed to constitute a new meteorite group, the Yamato-type (CY), based on their oxygen isotopic compositions and petrological features. The elemental compositional characteristics of the remaining six meteorites analyzed in this study, Y-86737 and Y 980134 (with CI affinities), and Y-86770, Y-86771, Y-86772 and Y-86773 (with CM affinities), suggest that these meteorites are all classified into the same chemical group CY. Based on the abundance of moderately volatile elements Mn and S, the twelve meteorites can be divided into two groups: one with levels similar to non-Antarctic CI and the other with intermediate levels between CI and CM. These results suggest that CY chondrites originate from two distinct parent bodies. To facilitate further discussions on CY chondrites, we propose naming the groups with compositions close to CI and CM as CYi and CYm, respectively.